1. Technical Field
The present invention relates to a Ni-based casting alloy and a steam turbine casting part using the same.
2. Background Art
In recent years, the development of thermal power plants with steam temperature of 700° C. or higher (A-USC, Advanced-Ultra Super Critical) has been advanced with the aim of increasing the efficiency of coal-fired power plants. For high-temperature parts of steam turbines that have been developed up to the present, iron-based 9Cr, 12Cr heat resistant ferritic steel, and the like have been used. However, as the upper limit of the steam temperature for heat resistant ferritic steel in the use environment is said to be 650° C., application of heat resistant ferritic steel to a 700° C.-level steam turbine is considered difficult. Therefore, application of a Ni-based alloy to a high-temperature part of a 700° C.-level steam turbine is envisaged. Many of Ni-based alloys are able to, when elements such as Al and Ti, as well as Cr, are added thereto and adequate heat treatment (i.e., aging heat treatment) is applied, have precipitated (i.e, precipitation strengthened) therefrom intermetallic compounds that are stable at elevated temperatures. Thus, high strength properties are exhibited at elevated temperatures. However, elements such as Al, Ti, and Nb that contribute to an increase in strength at elevated temperatures are problematic in that they will easily become segregated. However, for the materials of rotor shafts and the like, for example, it is possible to obtain a uniform part through casting by producing an ingot using a melting method that uses a double-melt process, such as VIM (Vacuum-Induction Melting)+ESR (Electroslag Remelting) or VIM+VAR (Vacuum-Arc Remelting), or a triple-melt process of VIM+ESR+VAR.
Meanwhile, steam turbine casings, steam turbine valve parts, and the like, which have large sizes and complicated shapes, are produced through casting with the use of large casting molds. However, as such casings and parts have complicated shapes, the aforementioned melting method is difficult to use. Further, when a casting method that employs a large casting mold is used, it is difficult to control the casting atmosphere. Thus, Al and Ti, which are active elements, may oxidize, or it might be difficult to control the components, which could result in defects and outside-of-specifications that would adversely affect the material characteristics of the obtained parts.
Accordingly, it has been considered to, even in the case of using the same Ni-based alloy for a large part to be produced through casting, apply an alloy that has been strengthened not through precipitation strengthening but through solid-solution strengthening, with the use of few active elements such as Al. As a candidate material therefor, Alloy 625 (Patent Document 1 and Patent Document 2) is known. The inventors prepared prototypes of thick specimens with the use of Alloy 625, supposing thick parts such as casings, and were able to confirm that the specimens had excellent manufacturability (without macro defects being generated therein) and had strength at elevated temperatures (i.e., have creep characteristics). However, when the prototypes were crushed for examination purposes, it was found that the grain structures were coarse and a large degree of micro segregation was present. In particular, with regard to micro segregation, it was found that there were large variations in alloy components between the dendrite core portion and the dendrite boundary portion, and some portions did not even satisfy a predetermined concentration. When the hardness was measured for the regions with such large variations in alloy components, some were found to be hard while others were not. Thus, such specimens are estimated to lack uniformity in strength. This may adversely affect a material that is required to be reliable over the long term, such as a steam turbine part, for example.
Patent Document 1: U.S. Pat. No. 3,046,108
Patent Document 1: U.S. Pat. No. 3,160,500